With a recent progress of portable electric equipment, demands for improving a capacity and enhancing an energy density are increasing against batteries serving as its power sources. In order to meet these demands, a nickel-metal hydride battery so-called "nickel-hydrogen battery" which utilizes MmNi.sub.5 system (Mm: misch metal comprising a mixture of rare earth elements such as La, Ce or Nd etc. ) hydrogen absorbing alloy as its negative active material, makes its advent recently and is expected to be put in practical use.
In the negative electrode (metal hydride electrode) of nickel-hydrogen battery utilizing alkaline electrolyte, the reactions expressed by equations (1) and (2) take place at time of charging and discharging. EQU H.sub.2 O+e.sup.- .revreaction. (H+OH.sup.- ( 1) EQU M+H .revreaction. MH (2)
(M: hydrogen absorbing alloy)
An electron is supplied from outside to cause a proton to be reduced to a hydrogen atom on a surface of alloy and absorbed in the hydrogen absorbing alloy when the charging is carried out. On the contrary, the absorbed hydrogen atom is ionized on the surface of alloy to release the proton when the discharging is carried out.
In the charge/discharge reaction of the metal hydride electrode, as described above, the alloy surface on which the ionization reaction (or reverse reaction) of hydrogen atom expressed by the equation (1) takes place plays an important part.
However, when charging and discharging ( oxidation and reduction) are repeated in the alkaline electrolyte, the MmNi.sub.5 system hydrogen absorbing alloy has such problems that surface corrosion of alloy progresses so as to inhibit the foregoing ionization reaction of hydrogen atom, increase a resistance between alloy particles (decrease an electronic conductivity) and gradually decrease the capacity to exhaust its service life.
In addition to the above problems, a sealed type nickel-hydrogen battery further includes such a problem that the hydrogen absorbing alloy of negative electrode is oxidized by oxygen gas generated from the positive electrode at time of over-charging, so that deterioration of alloy and shortening of service life are further accelerated.
In order to avoid the deterioration of alloy and shortening of service life due to the corrosion or oxidation of hydrogen absorbing alloy, there has so far been used a method for improving the corrosion resistance of the alloy itself by substituting a composition of MmNi.sub.5 system alloy i.e. a part of Ni with Al and Fe, Cu, Mn or Co etc. In order to further improve the corrosion resistance and oxidation resistance because of imperfection of this method, there have been proposed methods called as "microencapsulation method" in which corrosion resistant metal such as nickel or copper is coated on surfaces of hydrogen absorbing alloy (Published Patent Application (KOKAI) Nos. 61-64069 & 63-51051) and a method in which conductive agent of metallic powder or metal oxide is mixed in the hydrogen absorbing alloy in order to improve the electron conductivity between alloy particles, etc.
Although these methods have some effect to avoid the deterioration of alloy, a manufacturing cost is increased because they require troublesome processes such as the alkaline etching or the electroless plating and a capacity per unit weight of active material is decreased because a percentage of plating amount or amount of conductive agent is large which is unnecessary for the electrochemical capacity.
The electroless plating process for the microencapsulation of hydrogen absorbing alloy is composed of plural processes such as [1] Pretreatment of alloy, [2] Electroless plating, [3] Washing and [4] Drying etc. so that this process is troublesome and expensive in its manufacturing cost. In addition, heavy metal is included in waste liquid after the electroless plating so that pollution control facilities become necessary to cause an increase in the manufacturing cost. Further, in quality, it is difficult to control a plating amount and an uniformity of plating. Moreover, it is required to coat nickel or copper of at least 20 weight percents or more in order to effectively control the deterioration of hydrogen absorbing alloy by means of the microencapsulation method, so that a volume per unit weight of active material of negative electrode comprising the hydrogen absorbing alloy is minimized.
The alkaline etching process also includes troblesome problems similar to those of the microencapsulation method such as [1] Immersion in high-temperature alkaline liquid, [2] Washing for removing alkaline, and [3] Drying etc.
On the other hand, the method using the conductive agent is simple in manufacture and inexpensive in cost because the manufacturing process consists only of mixing the conductive agent into the alloy powder. However, 20 to 40 weight percents of conductive agent must be added in order to restrict the decrease in capacity of alloy, so that the same problem as above arises wherein the capacity per unit weight of active material becomes small. A conductive agent effective for maintaining the electronic conductivity between alloy particles for a long period is not found yet.
In order to put the nickel-hydrogen battery having a large capacity into practical use, it is required to increase an energy density of the nickel hydroxide electrode serving as the positive electrode. A sintered type plate has hitherto been used mainly for the nickel electrode. An upper limit of the energy density has been 400 mAh/cc, and it has been hard to increase the capacity beyond this limit. Recently, there has been developed a so-called pasted type nickel electrode in which nickel hydroxide powder forming an active material has been loaded in a metallic porous substrate having a high porocity, and its energy density has been improved up to approx. 500 mAh/cc.
In the conventional pasted type nickel electrode, however, it is indispensable to add a small quantity of cadmium to the nickel hydroxide powder forming the active material, in order to prevent swelling of electrode leading to a short-circuiting and a decrease in service life of battery. On the contrary, it is required to put the nickel-hydrogen battery, which does not include cadmium at all, into practical use in consideration of the environmental problem. Further, since the conventional nickel hydroxide powder is porous substance including a number of inner pores of particle, the battery capacity can not be increased up to 125% of that of conventional sintered type battery so far as the nickel hydroxide powder of such type including a number of inner pores of particle is used for the positive active material. Therefore, the conventional pasted type nickel electrode has included a problem to be solved from a standpoint of loading to a further high density.
The nickel-hydrogen battery using the foregoing conventional pasted type nickel electrode (including cadmium) and the metal hydride electrode have been unable to be formed into the complete pollution-free battery including no cadmium and have been expensive in manufacturing cost because they have been troublesome in manufacturing method.
The present invention is made in order to solve the above problems included in the prior atrs, and a first object of this invention is to provide a metal hydride electrode which prevents a decrease in capacity of hydrogen absorbing alloy, is simple in manufacturing process and inexpensive in manufacturing cost, and is excellent in charging and discharging performance.
A second object of this invention is to provide a nickel electrode which includes no cadmium and has a high energy density.
A third object of this invention is to provide a nickel-hydrogen battery which is simple in manufacturing process and inexpensive in manufacturing cost, is completely free from pollution, and has a high energy density.